FIELD OF THE INVENTION
The invention relates to a charge pump for generating an increased voltage that is above a supply voltage.
Charge pumps are used in electronic circuits where circuit elements are to be supplied or controlled with a voltage that is higher than the supply voltage of the overall circuit. Such voltages are needed in dynamic memories (DRAMs), for instance. The memory cell of a DRAM typically includes a capacitor that stores the information, to which access is gained through an MOS transistor that is typically of the n-conduction type and is connected as a transfer gate. The voltage of a word line connected to the gate terminal of the selection transistor must be above the supply voltage of the component, in order to enable the reliable writing of information that is to be stored in memory. Moreover, for reasons of reliability, the word line voltage should not exceed a maximum value.
From an article entitled "An Experimental 1.5-V 64-Mb DRAM" by Y. Nakagome et al. in the IEEE Journal of Solid-State circuits, Vol. 26, No. 4, Apr. 1991, it is known to generate the increased voltage by using a charge pump to charge a storage capacitor. The word lines are charged from the storage capacitor by charge transfer in the active phase and are brought to the full increased level with a smaller, two-stage charge slider. An n-channel MOS transistor is used as a switch in the charge pump, between the pump circuit and the storage capacitor. In order to control that transistor, voltages are used that are higher than the increased word line voltage being generated. Under unfavorable operating conditions, that can cause the destruction of the charge pump. Clearly, no precautions are taken to limit the maximum voltage. Moreover, the voltage at the circuit nodes between the n-channel MOS transistor and a sliding capacitor is frozen. If the supply voltage drops, the pump function of the charge pump can become inoperative even within the allowable fluctuation range, because the n-channel MOS transistor is no longer completely blocked.